Cyclic bit sequences can be used in a variety of monitoring applications, and they are particularly useful for determining a coarse location (e.g. a sector number) in an electromechanical data storage device. A typical disc drive, for example, includes a spindle motor rotating one or more discs at a constant high speed. Information is ordinarily written to and read from circular tracks on the discs through the use of an actuator assembly that includes a head that flies in close proximity above the corresponding surface of the associated disc. In a disc drive utilizing an embedded sector servo system, each track includes servo sectors that are separated by data sectors. Each servo sector includes a track identification code that can be unscrambled to determine a track number that uniquely identifies the track. Track identification codes are typically encoded in Gray code, a system for reducing the impact of an error due to a radial position change that occurs while reading a track identifier.
In a typical cyclic bit sequence, each servo sector also includes one bit that identifies an index location on the track. For example, one servo sector (the index servo sector) on each track can include a “1” in the index bit, while all other servo sectors include a “0” in the index bit. The index bit serves as the starting point for determining angular (or circumferential) position of a head relative to a data surface of a disc. Once the index bit has been encountered, the disc drive includes a counter that increments each time another servo sector is encountered. Thus, the index servo sector is often servo sector number zero; the next servo sector encountered by the head is servo sector number one, etc.
There are a variety of circumstances when it is desirable to know the circumferential position without waiting for the index bit to be detected, which may take an entire disc revolution or more. For example, drives with multiple discs frequently switch heads so that a different data surface can be read or written to. This may necessitate a detection of the position, unless angular alignment between the surfaces can be presumed. This may not be the case, for example, due to recent changes in manufacturing technology or due to a small displacement from a mechanical shock. The above-described bit sequence is also vulnerable to a read error or a point defect that may prevent a successful index mark detection.
Accordingly there is a need for better systems for determining a location by using a cyclic bit sequence, ones that create a more favorable combination of speed and robustness, yet maintain a suitable degree of format efficiency. The present invention provides a solution to this and other problems, and offers other advantages over existing systems.